Reported Magnesium (Mg) levels differ in enamel (normal vs carious), dentine, bone (normal vs osteoporotic) and in calcium phosphates associated with pathological calcifications (eg., B-TCP in dental and urinary calculi; ACP in soft tissue calcifications). Mg has also been implicated as a cariogenic element and as being 'preferentially' lost in incipient caries; in the inhibition of calcification and remineralization; and, in vitro, in the inhibition of the hydrolysis of DCPD and OCP to apatite. The overall objectives of this study are: (1) to define the nature of the association of Mg with biological apatites (whether surface-bound, a lattice substituent, or a constituent of a non-apatitic mineral, or of the organic matrix); (2) to determine the effect of Mg (alone and in combination with CO32-, F-, P2074-) on the formation of apatite and of 'precursor' calcium phosphates (DCPD, OCP, B-TCP, ACP) and on the transformation of these calcium phosphates to apatites; (3) to determine the limit of mg incorporation in apatite and the effects of other ions (Na+, CO32-, F-) on this limit; (4) to determine the critical Mg levels which will affect the formation (or non-formation) and stability of apatites and related calcium phosphates; and (5) to determine the effect of Mg on the physico-chemical properties (crystallinity, dissolution) of synthetic Mg-free and Mg-containing apatites and on Mg-substituted B-TCP and ACP. Synthetic apatites and related calcium phosphates* will be prepared from solutions (by precipitation or hydrolysis methods) and from gel systems. Biological materials will include: extracted human teeth (sound, carious, fluorotic); shark and fish teeth; bones (normal and diseased); dental and urinary calculi. Dissolution experiments will be in acetate buffer (0.1M KAc, pH 5, 37 degrees C) and the extent of dissolution expressed as mM Ca and mM P in the acid buffer after exposures for 10 to 120 min. X-ray diffraction, IR spectroscopy, TEM, thermogravimetry, atomic absorption and colorimetry will be used for characterization and analyses. Results from this study will provide new and significant information vital to the understanding of the role of Mg on the formation and stability of apatites and other biologically important calcium phosphates. [*DCPD - dicalcium phosphate dihydrate; OCP-octacalcium phosphate; TCP-tricalcium phosphate; ACP-amorphous calcium phosphate]